Many continental saline lakes are under the effects of salinity increase and anthropogenic eutrophication exacerbated by global change. The response of the food web to these drivers of change is not straightforward. To understand the consequences of salinity and eutrophication interactive effects on the food web, we studied the seasonal dynamics of zooplankton and phytoplankton and water quality parameters in 20 lakes of different salinity (from freshwater to hypersaline) and nutrient status (from oligotrophic to eutrophic) located in southern Siberia. We observed a pronounced bottom-up effect of nutrients, which induced an increase in the biomass of phytoplankton and zooplankton and a decline in water quality. A significant decrease in the species abundance of zooplankton was observed at a threshold salinity of 3 g L−1 and the disappearance of fish at 10 g L−1. The top-down effect induced by salinity manifested itself in an increase in the biomass of zooplankton with the disappearance of fish, and in the change of the size distribution of phytoplankton particles with an increase in the proportion of cladocerans in the zooplankton. Even though we observed that with the salinity increase the food web in saline lakes transformed from three-trophic to two-trophic without fish, we conclude that in the salinity range from 10 to 20–30 g L−1 this transition in most cases will not increase the ability of zooplankton to control phytoplankton. Interactive effects of salinity and eutrophication strongly depend on the size and depth of the lake, as deep stratified lakes tend to have a better water quality with lower biomasses of both phyto- and zooplankton. Thus, the salinity per se is not the driver of the decline in water clarity or the uncontrolled development of phytoplankton. Moreover, for deep lakes, salinity may be a factor affecting the stability of stratification, which mitigates the consequences of eutrophication. Thus, small shallow lakes will be the most vulnerable to the joint effect of salinity increase and eutrophication with the degradation of ecosystem functioning and water quality at moderate salinities of 3–20 g L−1.
The ability to change the reproduction mode and produce diapausing eggs, which is prevalent in many zooplankton species, significantly impacts on the evolution and ecology of aquatic communities. The production of diapausing eggs is controlled by multiple effects of biotic and abiotic factors, including infochemicals. We have investigated the effects of chemicals exuded by conspecifics and ecologically close competing congers, Moina brachiata and M. macrocopa, which coexist in the same water body, and by larger Cladocera species (Daphnia magna) on the change of reproduction mode, specific growth rate and fecundity of M. brachiata and M. macrocopa females. The production of gametogenetic eggs in both species was detected only in waters from crowded cultures of conspecifics. The water from crowded cultures of conspecifics reduced the specific growth rate of the juvenile females of both species that later switched to gametogenesis. While it either did not affect (in M. macrocopa) or even increase (in M. brachiata) the specific growth rate of the juvenile females that later reproduced by parthenogenesis. Females of M. macrocopa released significantly fewer neonates in the water from crowded cultures of conspecifics than in all other treatments, while the fecundity of M. brachiata females was the same in all treatments. To understand the phenomenon of diapause induction under the effect of chemical cues in zooplankton, a link between laboratory tests and ecological research should be established, and the chemical composition of the signals should be determined.
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